1. Field of the Invention
This invention generally relates to power amplifiers, and in particular, to dynamic biasing of power amplifiers.
2. Related Art
In today's society, both the presence and use of communication systems are increasing at a rapid pace and wireless and broadband communication systems and infrastructures continue to grow. This acceleration has created a strong and ever-growing market for electronic equipment that employs more powerful, efficient, and inexpensive communication components.
Electronic equipment such as computers, wireless devices, broadband devices (i.e., standard telephones), radios, televisions and other similar devices may communicate with one another by passing transmission signals through free-space (i.e., air and space) and through guided media such as wire, cable, microwave, millimeter wave, sonic, and optical connections. These transmission signals go through various processing steps during their communication. One such processing step involves amplifying the transmission signals.
FIG. 1 is an example amplifier transfer function plot 100 of output voltage 102 versus input voltage 104 with an amplification curve 106 that graphically illustrates the typical linear amplification process. In FIG. 1, an input signal 108, having input amplitude 110, is linearly amplified to an output signal 112 having amplified output amplitude 114. If the amplifier gain is one (“0 decibels” also known as “0 dB”), the output amplitude 114 will be of the same magnitude as the input amplitude 110. If the amplifier gain is greater than one (a positive value in dB), the output amplitude 114 will be greater than the input amplitude 110. If the amplifier gain is smaller than 1 (a negative value in dB), the output amplitude 114 will be less than the input amplitude 110. If the amplifier operates in a mode that provides good linearity, then an increase in the amplitude of signal 110 in a given proportion will result in an increase of signal 114 in the exact same proportion. This mode of operation, however, generally requires a higher level of current supply, and thus tends to make the amplifier less energy efficient. For the amplifier to operate in a mode that yields good efficiency, it is required generally that the current consumption be lower. This, however, generally causes the amplifier to reach output signal compression earlier, meaning that for high levels of the output signal, the amplitude of signal 114 cannot increase in the same proportion as signal 110, but instead will have a smaller amplitude increase.
Thus, in amplifying these transmission signals, the power amplifiers within the electronic equipment (such as the type utilized in current commercial applications such as wireless handsets and the like) typically suffer a tradeoff between efficiency and linearity. According to this tradeoff, improvements in linearity are typically achieved by sacrificing the efficiency of the power amplifier through increased biasing.
As an example, FIG. 2 shows an example conventional amplifier 202 within an electronic device 200. The amplifier 202 is typically utilized to increase the power of an input transmission signal 204 from its original power level at an input 206 of the amplifier 202 to the desired power level of an output signal 208 at an output 210 of the amplifier 202. For an input transmission signal 204 having a low power level, the amplifier 202 generally receives sufficient bias current from a power supply 212 and from the biasing circuit 216 of the electronic device 200 to operate. It is appreciated by those skilled in the art that as the power level of the input transmission signal 204 increases, the amplifier 202 may require additional bias current from the power supply 212 and the biasing circuit 216 to operate properly. However, at higher power levels, the circuitry (not shown) that delivers the bias current from the power supply 212 and the biasing circuit 216 to the amplifier 202 may not be able to supply the higher bias current to the amplifier 202, due to hardware limitations in the circuitry.
A known approach to reduce the effects of this problem is to utilize a biasing circuit 216 that provides the amplifier 202 with a higher nominal bias current intensity via signal path 218. However, this approach tends to increase the current consumption of the amplifier, and thus degrade its energy efficiency at lower power levels.
Although this additional bias current enables the amplifier 202 to extend its linear amplification operation as power increases, the amplifier 202 may still experience compression at the highest power levels. When the amplifier 202 experiences compression, its actual output is less than a desired output. For example, if the amplifier 202 is designed to give a gain of 5 decibels (“dB”) to a transmission signal 204 but only gives 4.5 dB, the amplifier 202 may be characterized as experiencing a compression of 0.5 dB. It is appreciated by those skilled in the art that extreme input transmission signal 204 power levels may actually cause the amplifier 202 to severely distort the signal and totally compromise the integrity of the information contained in that signal, beyond any possibility of recovering the data at a receiver.
When the power level of the input transmission signal 204 reaches a threshold value (typically known as the amplifier “gain compression point”), the compression of the amplifier 202 reaches a point at which it is more efficient, but less linear. Therefore, there is a need to extend the amplifier gain compression point to a higher output power level and improve the tradeoff between efficiency and linearity in a power amplifier.
FIG. 3 shows an example implementation of an electronic device 300 utilizing a known approach for extending the amplifier gain compression point to a higher output power level and improving the tradeoff between efficiency and linearity in an amplifier 302 utilizing a technique generally known as dynamic biasing. In dynamic biasing, the level of biasing is determined responsive to the amplitude of a radio frequency (“RF”) signal 304 at the input 306 of the amplifier 302. As the amplitude changes, so does the level of biasing. A typical approach to dynamic biasing involves detecting the envelope of the RF signal 304 (through a diode-based circuit for example) and biasing the amplifier 302 as a function of the RF signal 304 envelope. This way, the biasing level is kept to a minimum at low power levels, and is allowed to automatically adjust at a higher level as the RF signal power increases, thus optimizing the energy efficiency at low power levels and improving the efficiency/linearity trade-off at higher power levels.
An external detection circuit 308 (i.e., a circuit external to a biasing circuit 310) is utilized to detect the envelope of the RF signal 304, via signal path 312, and provide the necessary information for linearity correction and efficiency control to the biasing circuit 310. Additionally, the external detection circuit 308 may also optionally detect the envelope of the RF output signal 314, via signal path 316. The biasing circuit 310 then provides the necessary biasing current, via signal path 318, to the amplifier 302.
However, a problem with this approach is that it may consume an excessive amount of semiconductor chip space. This problem in this approach is that the external detection circuit 308 is external to the biasing circuit 310, and may need temperature compensation circuitry (not shown) and pre-biasing circuitry (not shown). The temperature compensation circuitry compensates for temperature variations, and the pre-biasing circuitry is often required to place the external detection circuitry 308 in the necessary state of sensitivity. Additional problems with this approach also include excessive cost and complexity.
As a result, there is also a need to extend the amplifier gain compression point to a higher output power level and improve the tradeoff between efficiency and linearity in an amplifier utilizing a dynamic biasing system that is not external to the biasing circuit.